Human Pigmentation

Question 1

Plasticity/Adaptability

Answer 1

The ability of an individual organism to short-term make physiological changes in response to exposure to stressful environmental conditions

• often associated with changes that come to be over the period of growth and development

Question 2

Adaptation

Answer 2

The ability of an individual organism to make anatomical or physiological changes in response to exposure to stressful environmental conditions

• often associated with changes that come to be by altering patterns of growth and development

Question 3

Acclimatization

Answer 3

State reverts after environmental stress is removed

Question 4

What is the largest organ of the body?

Answer 4

Skin
- our most conspicuous polymorphism

Question 5

The many functions of Skin:

Answer 5

• protection
• thermoregulation
• Vitamin D synthesis

Question 6

What is the pigment primarily responsible for light-dark coloration of most animals?

Answer 6

Melanin

Question 7

All people have the same number of Melanocytes, but what are melanocytes?

Answer 7

Cells that make melanin

Question 8

Skin color depends on how much and which type of _____ is produced

Answer 8

Melanin

Question 9

Eumelanin

Answer 9

molecule responsible for most variation in human skin pigmentation
- Dark: Brown to black
- Concentration accounts for skin’s darkness

Question 10

Pheomelanin

Answer 10

• Lighter: yellow to reddish brown
• Accounts for skin’s reddish-brown color

Question 11

Lightly pigmented people are while (the color of connective tissue)

Answer 11

A pink hue comes from blood hemoglobin showing through

Question 12

Our skin has a layer structure. Melanocytes are near the base of the epidermis

Answer 12

• Melanocytes are embedded between the basal layer and the overlying stratum spinosum
• As melanosomes fill up with melanin, the pigment gets distributed to new skin cells through the dendrites

Question 13

Skin pigmentation is determined by:

Answer 13

• rate of production
• distribution
• size of melanin packets

Question 14

There are a lot of biological factors involved in melanin and skin pigmentation

Answer 14

• some are genetic
• some are hormonal

Question 15

Are all babies born with lighter skin than they will have as adults?

Answer 15

Yes; because melanocytes have to “ramp up” into production

Question 16

Human biologists measure skin color using spectrophotometers or similar equipment

Answer 16

This measures how much light is reflected at a specific wavelength
- light colors reflect more light
- darker colors absorb more light

Question 17

Human skin pigmentation is an excellent example of a continuous and complex phenotype

Answer 17

• skin pigmentation is highly polymorphic
• continuous: the polymorphisms have gradient-style variation (there are an infinite number of intermediate states)
• complex phenotypes are influenced by environment (genetic-adaptation=constitutive pigmentation; acquired-acclimatization=facultative pigmentation)
• skin pigmentation is polygenic (many genes)

Question 18

Is skin pigmentation highly heritable?

Answer 18

Yes
- BUT the genetic control over the phenotype is not straightforward

Question 19

The TYR gene makes the enzyme tyrosinase, which is involved in _____?

Answer 19

melanin production at a few different stages

Question 20

Albinism

Answer 20

caused by the lack of melanin
- over 100 distinct genetic causes/mutation in TYR

Question 21

4-6 genes identified for skin pigmentation genetics?

Answer 21

1.) TYR: Tyrosinase
2.) MC1R: Melanin production
3.) OCA2: P protein, has to do with melanosome function
4.) OCA4
5.) SLC24A5

Same reflectance value from different genotypes
- some alleles can be MASKED

Question 22

Some genes involved in skin pigmentation are also involved in eye and hair color

Answer 22

Melanin is also involved in eye color
- eye darkness vs. lightness

OCA2 (Chromosome 15) was identified by earliest studies to be the dominating factor for eye color determination
- now known that other genes are involved (HERC2) that don’t affect melanin directly

MCR1 contains regions where the allele present is associated with whether or not a person has green eyes

TYR can stop melanin production at the highest hierarchical level

Question 23

UVR

Answer 23

Energy with wavelength between visible light and x-rays
- has potential to alter biological processes

Question 24

UVR can be sub-divided into:

Answer 24

UVA: longest wavelength, potential to penetrate skin deeply
UVB: intermediate wavelength, potential to penetrate skin less deeply
UVC: shortest wavelength, atmosphere absorbs/scatters

Question 25

More and larger melanosomes permit___

Answer 25

less UVR penetration

Question 26

Darkest human pigmentation can block ca. 95% UVR

Answer 26

Light pigmentation blocks less than 50% UVR

Question 27

UVR negative effects

Answer 27

- UVR can damage the skin - sunburn (caused especially by UVB) - destroys skin elasticity (UVA) - can damage molecules and compounds inside the body - all UV wavelengths can damage DNA - This is one cause of melanoma

Question 28

Short-term, reversible response to UVR penetration

Answer 28

Tanning

Question 29

Tanning

Answer 29

When UVR exposure exceeds a threshold, your body starts a self-regulation mechanism - existing melanin skin darks (UVA) - melanin production is increased (UVB) (at this point, you already have DNA damage)

Question 30

All people can tan. All people can burn.

Answer 30

TRUE; the amount you see the different will depend on how dark someone is

Question 31

Folate (folic acid)

Answer 31

B vitamin required to produce nucleic acids - an process involving cell proliferation Linked to many birth defects - plays an important role n brain and spinal cord Required for spermatogenesis Myelin and serotonin - brain function

Question 32

Positive effects of UVR

Answer 32

Catalyze production of compounds inside the body Vitamin D synthesis - UVB exposure initiates pre-vitamin D production

Question 33

UVR intensity varies with latitude and altitude

Answer 33

TRUE

Question 34

Earth is a sphere

Answer 34

Cross-sectional area of "a ray of sunlight" increases with distance from equator Earth has a thick atmosphere that absorbs and scatters UVR - a ray of sunlight hitting surface at equator passes through less atmosphere than one hitting surface at high latitude - altitude alos affects UV intensity for this reason Intensity of UV radiation increases near equator, and creates at high altitude localities near equator

Question 35

Human populations living in some parts of the world are at risk of UVR overexposure while...

Answer 35

others are at risk of underexposure

Question 36

At high latitudes

Answer 36

Season affects UVR - each currently has an axial tilt of about 23 degrees - Axial tilt accounts for variation in length of daylight (near the equator, there is no significant variation in day length throughout)

Question 37

Overexposure risk in the summer and underexposure risk in the winter example

Answer 37

True

Question 38

There is a very strong correlation between pigmentation and UVR strength... how? Why?

Answer 38

Pigmentation in the skin serves as a natural defense mechanism against the harmful effects of UV radiation. The darker the skin, the more melanin present, providing greater protection against sun damage

Question 39

Non-human apes are lightly pigmented under their hair (they have little melanin in their skin)

Answer 39

- they can develop facultative pigmentation on their more exposed areas - or exposed areas are permanently pigmented

Question 40

The evolution of darkly pigmented skin is probably linked to hairlessness

Answer 40

- responses to increased temperature regimes with expansion into shadeless environments - hair prevents effective evaporative cooling

Question 41

When did hominins get naked?

Answer 41

When they moved into shadeless environments - sometime near the beginning of the Pleistocene Homo erectus - shows adaptation to hot environments in body proportions - associated with dry and open grassland fauna - researchers propose that the species was dark and naked

Question 42

How would darkly pigmented skin have contributed to differential fitness?

Answer 42

Proposed hypotheses: - sunburn - folate protection - skin cancer

Question 43

The folate hypothesis

Answer 43

Argues that protection of folate stores enhanced fitness of early hominins with darkly pigmented skin - folate depletion in females is associated with the development of neural tube defects in the fetus - darkly pigmented women have lower incidence of neural tube related birth defects - there is an annual peak of babies born with neural tube defects that were conceived in the summer months - in men, folate deficiency is associated with impaired spermatogenesis

Question 44

How would lightly pigmented skin have contributed to differential fitness?

Answer 44

- Darwin thought it was sexual selection - increased vitamin D synthesis

Question 45

Support for the Vitamin D hypothesis

Answer 45

Vitamin D and bone formation - Calcium absorption - osteoblast and chondrocyte activity - can lead to rickets Vitamin D vital to immune system function Independent evolution of lightly pigmented skin - molecular signature of purifying selection